PS401 - Attack/Release Opto-VCA

PS401 Attack/Release Opto VCA

The Dynamics of synthesized sounds are controlled with a Voltage Controlled Amplifier, or VCA. In a classical subtractive synthesizer arrangement the VCA is controlled through an envelope generator, allowing more interesting dynamic possibilities than just the on/off of a note gate.

Vactrol based Opto-VCA add an Attack and Release time automatically due to the relatively slow turn on, and off times of the led-ldr pair inside a vactrol. The ”opto” part of our VCA is a fast and accurate AC-Optocoupler, which adds a tiny bit of Attack time and Release tail to the signal, but not so much as a Vactrol would. We Decided to go with the optocoupler since factory made Vactrols are getting more rare and expensive, and altho rolling your own is a good choice, the response varies due to led and ldr types and manufacture tolerances.

The Paper Synthesizer PS401 adds a bit more control to the opt VCA with Attack and Release control potentiometers. If both turned to zero, the module acts like a normal VCA. The module comes also with an bipolar offset potentiometer for manually controlling the amplitude, or offsetting the control voltage lower or higher. Since this adds the possibility to drastically over amplify the signal hotter than the eurorack standard 10Vpp, we added an adjustable ”clip led” to show when your signal is getting too hot to handle.

Powered with Eurorack standard +-12 volts or with +-15 volts Uses Eurorack power sockets and interconnect cables Power consumption: Designed for XXhp panel width. Does not include Faceplate nor potentiometer knobs.

Theory of operation

Audio & Video








14p ic socket 1
8p ic socket 1
16p IDC male 1



pinstrip 90 deg 4 pins
Thonkiconn 3.5mm jack socket 3

Scope Pictures

Sources of information